Many products are packaged in trays which are overwrapped with a clear, plastic, heat shrinkable film. The clarity of the film adds to the market appeal and contributes to the ultimate success of the product sales. It is also important to obtain a wrapped package in which the plastic film is smooth in appearance, rather than wrinkled, so as to optimize the appeal of the product in the eyes of the customer. These basic concepts particularly apply to packages wrapped in heat shrinkable film.
In the process of wrapping and shrinking of film, it is common to seal the film between successive packages on a line crosswise of the conveyor direction of travel and to subsequently heat shrink the film into intimate, wrinkle-free contact with the product and the product-carrying tray. During ordinary film shrinking operations, a blast of heated air is directed at the wrapped product so as to shrink the film. Such a system using heated air to shrink a wrapping film is described in applicant's prior U.S. Pat. No. 5,062,217 for a Selective Sequential Shrinking Apparatus And Process. The teachings of U.S. Pat. No. 5,062,217 are incorporated herein by reference.
It has also been known to shrink heat shrinkable film by use of hot water. However, so far as applicant is aware, it has not been known to use both hot air and warm or hot water in a combination as in the invention.
The hot air method of the '217 patent represents a significant improvement over the prior art, but through practice has also been found to have limitations under certain circumstances. Particularly difficult circumstances are encountered in the shrinking of heat shrinkable film around a cold product such as, for example, tray-packaged poultry. In order for heat shrinkable film to shrink, it is necessary for the film to achieve a certain minimum temperature. It has been discovered that when the heated air impinges on the upper portion of the heat shrinkable film, the film is pressed downward into contact with the cold poultry product. The cold poultry, in these circumstances, tends to absorb a significant portion of the heat from the heated air and prevent the film from getting hot enough to shrink. Because of this heat absorbtion, the film frequently does not shrink in those sections of the film which are in contact with the cold poultry product but does shrink where not in such contact, creating a differential shrink situation and which produces undesired wrinkles.
If in order to overcome this undesired partial shrinkage, an attempt is made to increase the air temperature or the time during which the film is exposed to the heated air other problems have been found to occur. One such problem is that the added heat exposure may cause a hole to be burned through the portion of film which is not in contact with the cold poultry product thereby causing defective packaging. Another problem which occurs with regularity is that when the poultry comes in contact with either hot air for a greater than normal time or with hot air at an excessively high temperature, the poultry product begins to be cooked, which condition is also unacceptable.
A further difficulty frequently occurs as a result of heat shrinking the film with hot air in the fast-moving conveyor driven production equipment of today. Heat shrinkable film wrapped packages with their ends heat sealed may, for example, be fed from the wrapping machine to a shrinking machine at a processing speed of 40-50 pieces per minute. Immediately after sealing at this speed, the seam is quite warm and fairly weak. At this processing speed, it is also to be observed that there is only 1 or 2 seconds available for the sealed ends to cool and solidify before the beginning of the shrink process. The outcome is that, under certain conditions of speed, hot air heat and film, some of the end seals will rupture when subjected to the stress of heat shrinking.
Therefore, an objective of the present invention is to provide an apparatus and method for shrinking of heat shrinkable film wrapped and sealed around a product so as to avoid wrinkles.
An additional objective of the present invention is to provide an apparatus and method for shrinking of heat shrinkable film so as to prevent film burn-through or product cooking.
A further objective of the present invention is to provide an apparatus and method for shrinking of heat shrinkable film without rupturing of previously welded end seals.
These and other objectives will become apparent from the details of the disclosure which follows.